Speaking at a news conference in Mexico City Thursday, Obrador said he plans to take the matter to the Federal Electoral Tribunal, which would have until September 6 to declare a winner in the election. He also called on his supporters to gather for a rally in the city's main square on Saturday.

Official results issued early Thursday indicate conservative candidate Felipe Calderon's lead is just four-tenths of a full percentage point over Obrador.

Obrador has demanded that electoral officials count the ballots one-by-one, instead of tallying vote totals as they have been doing. He said he will make the same request to the tribunal.

Calderon's supporters are celebrating in the streets of Mexico City in anticipation of their candidate being declared the winner.